Orthotropic Steel Decks (OSDs) are widely used in various types of steel bridges due to their benefits of light weight, high load bearing capacity and speedy construction. Although many improvements in aspects of design, fabrication, inspection, and maintenance have been achieved over the years for such bridge decks, fatigue remains a predominant problem, mostly because of the complexity of prediction methods. Many researchers have tried to investigate this component through experiments. However, performing only experiments may not lead to a cost-effective solution. Therefore, it is necessary to combine the experimental data with the numerical approaches. Particularly Linear Elastic Fracture Mechanics (LEFM) allows to model and analyse the crack propagation until subsequent failure, and significantly reduces the requirement of experiment. ABAQUS® provides an enriched feature, commonly referred to as the Extended Finite Element Method (XFEM) which incorporates two enrichment function namely the discontinuity function which represents the gap between the crack surface and asymptotic function which captures the singularity and thus can be used to model discontinuity independent to the finite element mesh. To evaluate the modelling efficiency and validate the simulation methodology, two XFEM-model based on LEFM and Virtual Crack Closure Technique (VCCT) are developed and the simulated results are compared with the experimental data. The first phase of the thesis deals with the numerical simulation to investigate the crack propagation rate in Compact-Tension (CT) specimen for different stress ratios. The results of two-dimensional (2D) model are found to be in good agreement (within 1.48%) with the fatigue coupon test results. As most of the work concentrates on 2D shell model, the extension to three-dimensional (3D) solid requires the investigation of related parameters to consider through-thickness effects. Nevertheless, the mechanism of 3D model is studied, and the simulated results match with the 2D results for fatigue crack growth (a, N). Moreover, a reliable technique of computing Stress Intensity Factor (SIF) is obtained by comparing with the ISO 12108 standard formulation. However, when the SIF and fatigue crack growth are combined, the crack propagation rate in 3D is overestimated (about 26%) when compared to the experimental data possibly because of the imperfection in the application of boundary conditions. The second phase deals with the numerical simulation in welded connection of OSD to determine the Paris law constants (C, m) by correlating the numerical result of fatigue crack growth with the beach mark measurements obtained in the fatigue experiments. Prior to automated XFEM simulation, a set of finite element analyses are performed to determine the vertical deformation, longitudinal stain distribution and hotspot stresses to validate the numerical model as per the test setup. The results of numerical analyses showed a good correlation (within 18%) with test data and Paris law constant C is predicted to be lower than the recommended value by IIW standard. The validated methodology is then applied on large scale to an existing bridge (Suurhoff bridge) structure which was built in 1971. In this case study, a crack length of 230 mm was detected in the deck plate originating from the root of the stiffener-to-deck plate welded connection between the cross-beams using TOFD measurements. To verify the problem, a numerical model is developed based on the dimension of the bridge to evaluate the crack initiation period and the crack propagation period. The crack initiation period is predicted using hotspot stress method and the crack propagation period is evaluated using automated XFEM simulation. Overall, the total fatigue load cycles are predicted to be 7.86 million which is equivalent to 48 years. A similar crack length was however detected after a service life of 44 years. This overestimation can be possibly explained as the model did not take residual stresses and other welding defects into account. The numerical model showed a good correlation with the real scenario and is therefore used to predict the permissible limit of deck plate crack length of 500 mm. The model predicted 8.02 million load cycles for a crack length of 500mm, which is equivalent to 34 years after the crack initiation period. Nevertheless, the fracture mechanics approach showed improvements in the assessment of fatigue life.

The interest in high strength steel (HSS) shows an increasing trend mainly due to its high yield strength, low weight to strength ratio, decreasing costs of the base material and fabrication. The proposed 2020 version of the Eurocode 3 part 1-8, still under discussion at the time of writing of this thesis, covers steel with yield strength up to 700 MPa. This standard has recommended high material reduction parameter for joints with steel grades above S355 to S700. This is done to consider the low deformation observed for steel between 450 MPa and 460 MPa, to include the insufficient knowledge of the material properties of HSSs and the impact of these properties on the current standards. Secondary bending stresses are omitted for the static design strength of mild strength hollow section joints. But for high-strength joints rules regarding these stresses need to be re-analyzed, since the impact of HSS on hollow section joints may be different. Goal: In this study the main goal is to investigate the level of the secondary bending stresses in hollow section joints made of high-strength steel using numerical analyses. Furthermore, the magnitude of the material reduction factors recommended by the proposed 2020 version of the Eurocode 3 part 1-8 are evaluated. An attempt is made to validate the FEM against test data. The commercial finite element program ABAQUS® is used to build the FEM. Roughly good agreement between the test and the FEA is obtained. A parametric study is performed on an isolated gap K-joint. The joint geometries, boundary and loading conditions are taken from the literature. To study the impact of the material parameters on the secondary bending stresses in RHS joint, various parameters including material properties, gap size, brace width to chord width ratio and weld type are varied. These parameters influence the stiffness, strength, stress distribution and secondary bending stresses. A total of 32 gap K-joints is analyzed in this research. Also, an attempt is made to obtain the ultimate load resistance using the yield line theory. As results, valuable information about the joint behavior of high-strength hollow section joints is obtained. It is found that secondary bending stresses cannot be neglected when high-strength steels are used.

More offshore wind projects are being developed. Shallow locations are more occupied and a tendency to deeper water and to larger wind turbines is seen. These create more bending moment which can be more easily resisted by jacket substructures. The state of the art installation process of a jacket is as follows. The foundation piles are driven into the seabed. The jacket leg is installed by sticking a stabbing cone into the stick-up length of the foundation pile and fill the space between the cone and the pile with grout. In this situation a lot of grout, steel and installation time are used. An alternative connection could be a wedge connection which is considered in this thesis. A wedge connection consists of four parts: jacket leg, connector element, wedges and foundation pile. The challenge to apply this connection to jackets is to overcome the installation tolerances. Due to the inaccuracy of the driving of the foundation piles, the holes in the jacket legs and the foundation piles do not align and the wedges cannot be installed. The installation tolerances consist mainly of center-to-center distance, pile verticality and a vertical tolerance. The maximum translational tolerances are 75 millimeter while the maximum rotational tolerance is maximum 0.83 degrees. 23 different concepts have been developed to deal with the installation tolerances. All the concepts have some drawbacks and do not fully solve the problem without downsides. The magnitude of the installation tolerances is therefore reduced and from a multi-criteria analysis a screw thread connection is proposed. The advantages of the screw thread connection are: a reduced amount of material, no grout is required and the installation time of the jacket to foundation pile connection can be reduced up to 75 percent. The screw thread connection overcomes the vertical tolerance and the tolerance around the longitudinal axis of the foundation pile by rotation of the connector element. In case of an pile inclination, the connector element and the foundation pile do not fully align and a gap remains in between them. By applying a preloading force to the connection it is still not possible to close this gap. The space between the connector element and the foundation pile has to be filled with an epoxy resin. The screw thread connection does fulfill the requirements for the ultimate limit state based upon hand calculations for net stresses, bending stresses and contact stresses. The fatigue limit state of the wedge connection meets the requirements. Only the fatigue limit state of the screw thread does not fulfill the requirements based upon a hand calculation. The three largest issues of the screw thread connection are the fabrication of the screw thread profile, the fatigue in the screw thread and the installation of the epoxy resin between the connector and the foundation pile. The first step to develop the screw thread connection further is an more extensive investigation to these three issues.

The stay cables of the north bridge of the Galecopperbrug are reaching the end of their life span. Rijkswaterstaat, which is responsible for the bridges, needs a reliable Structural Health Monitoring (SHM) method to monitor the current state of the stay cables. At the moment, Rijkswaterstaat experiences issues with defining the state and the residual life span of the stay cables of the
Galecopperbrug. This study focuses on the technique of Acoustic Emission (AE) for monitoring the stay cables of the Galecopperbrug. The main research question of this study was: “Is the AE-system used in a fracture-based assessment suitable for structural health monitoring of the stay cables of the Galecopperbrug?”. In this study the following methods were used to investigate the AE behaviour of stay cables: a literature study on previous research and the current knowledge of AE and the structural behaviour of cables was done, a full-scale and two verification experiments were performed to investigate the AE behaviour related to wire breaks and to investigate the accuracy of linear source location techniques, an analytical model of the stay cables was made to investigate the influence of different parameters to the capacity of the stay cables and to predict the stress distribution in the wires during the experiment and finally, a SCIA model of the Galecopperbrug was used to investigate the load bearing distribution of the Galecopperbrug. In this study, it was found that wire breaks inside (stay) cables will generate elastic stress waves which can be captured and recorded by AE sensors. Based on an experiment where multiple wire breaks occurred, it was shown that wire breaks can be identified with the help of AE techniques. However, the identification of wire breaks is mainly depending on the correct choice of sensor type. Based on the experiments and assumptions that were made in this study the R6I-AST type of sensors are more suitable for wire break detection and the R3I-AST sensors are more suitable for AE signals due to impacts. Based on the assumptions and the experiments performed in this study it can be concluded that AE can be used in a fracture-based assessment for SHM of the stay cables of the Galecopperbrug.